"Alicia [Silverstone] was so young at the time. She was a hippie girl running around with dogs in her sweatpants," says Clueless costume designer Mona May. "Here we are putting her in all the designer clothes in the world." The beloved film turns 20 this year, and two decades later the costume designer who made Alicia Silverstone synonymous with yellow tartan and gave Azzedine Alaia a new significance is reminiscing about the fashion forward film. "When Alicia put the clothes on, everything changed. She started walking differently, moving her head differently, flipping her hair. It's what we do as costume designers, we give them the tools. There were sixty changes for each of the girls," the blonde, dressed in a bright red pantsuit and an off-kilter fedora, admits. "The whole fashion is in a way aggressive. We wanted to push it."

Being a huge fan of the film — I even lived life as Cher Horowitz for a day — I was oozing with questions to ask May about the movie's many looks, but had to begin with the film's most iconic ensemble: The yellow plaid. And it turns out, her inspiration for the outlandish getup was rooted in reality. "You start with: What would the girls' first day of school outfit be? The quintessential plaid skirt," she says. "Then you want to take that to the next level. So you say a suit would be much better. Then what color? Yellow pops. Red would be probably not right. [Alicia] tried it on and it looked amazing."

With Alicia Silverstone in yellow paid, what would BFF Dionne, played by Stacey Dash, wear? "I ended up making the other suit for Dionne with the vinyl lapel," she explains. "It's looking at what the girls would wear together for the first day. Call her up, 'OK, I'm wearing my yellow, so you're wearing your black and white.' And of course Dionne gets her crazy hat to go with it because she's funky and cool. Dionne was bolder, more sophisticated — she knew more about fashion. Cher gets her furry backpack. We add more edgy things, like the over-the-knee stocking with the Mary Jane's. It combines and works together to go, 'Bam! This is right.'"

Seeing the film for the first time as a young girl, I couldn't help but feel Cher's little red dress — the one she wore to seduce Christian — was the epitome of sexy. "It's all about simplicity," May says of the dress. "It's the spaghetti straps, it's flimsy. It looks like it might fall off easily. Those are the things that are subliminal. We never wanted to be overly sexy for her. It was never about a push-up bra, it was never about a corset, it really was the innocence and flirtation. It's about soft, silky."

For Christian, the always well-dressed and eternally cool guy pal of Cher's, May opted for a timeless classiness. Her inspiration? "Definitely The Rat Pack," she says. "Almost a mixture of 1940s and '50s. It suited his body really well too. We were thinking about Marlon Brando, when he wore just a white T-shirt and it was perfect. The first time we see Christian walking into the classroom it's sexy: A white T-shirt, the high-waisted pants, he throws his jacket over his shoulder. It's timeless. He could be in the movie now and you'd probably be swooning the same way."

The designer, whose credits include Never Been Kissed, The Wedding Singer, The House Bunny, and so many more, found the biggest challenge in the sheer magnitude of bodies she had to clothe. "Girls are always more challenging [to dress]. Guys are like, 'Oh cool, let me try that on, that's fun!' Like Murray with all his crazy hats, or Elton with his metro cool, designer [clothes]. But the girls [are harder]. They have insecurities, you want to find out what looks good on them," she says, admitting: "There was whimsy and silliness to it. We weren't super serious about it. It was playful, it was about the process. It looks so amazing, but to get there? To get sixty outfits, how many fittings do you have to have? How many accessories do you have to have?"

It may have been two decades since she dressed the students and faulty of Bronson Alcott High, but she still keeps in touch with many members of the cast and crew. "I just did a movie with Alicia, it was a small movie in Chicago so I prepped all her clothes. We had no money so it was me and Alicia and thrift stores on Melrose."

Much to May's delight, a Clueless Broadway musical is now in the works. "It's my next adventure with Amy [Heckerling]!"

The 20th anniversary edition of Clueless is now available on Digital HD.